The influence of both the nickel-support interaction and the Pd promotion on the activity and thiotolerance during the hydrodesulphurisation (HDS) of dibenzothiophene (DBT) was studied in a variety of amorphous silica-alumina-supported nickel catalysts (Ni/ASA). The catalysts were characterized by means of X-ray diffraction (XRD), N-2 adsorption-desorption measurements. temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, CO chemisorption and X-ray photoelectron spectroscopy (XPS) techniques. In the low-nickel-content catalysts, a difficulty in reducing the Ni2+-support phase was observed, whereas in the high nickel-content catalysts a major reducible NiO phase was developed. Catalysts containing low amounts of nickel exhibited higher activity and selectivity toward ring-opening (RO) products than those containing higher nickel loadings. The larger ring-opening activity and the greater resistance to deactivation of the low nickel-loading catalysts appear to be related to the morphology of the nickel phases at the catalyst surface, consisting of very small Ni clusters separated by non-reduced Ni2+ species. The high activity of the catalysts in the ring-opening reaction can be explained assuming that non-reduced Ni2+ species might to force DBT adsorption on Ni-0 phase via C=C bonds perpendicular to the surface. In addition, the greater resistance of the catalysts to deactivation was related to H2S adsorption on Ni2+ species, thus maintaining Ni-0 locations available for performing the ring-opening reaction. (C) 2003 Elsevier Science B.V. All rights reserved.